This application claims priority to an application entitled xe2x80x9cMethod of Improving Receiving Performance for Network-Aided GPS Receiverxe2x80x9d filed in the Korean Industrial Property Office on Nov. 3, 2000 and assigned Ser. No. 2000-65179, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a global positioning system (GPS), and in particular, to a method of increasing the sensitivity and performance of a network-aided GPS receiver.
2. Description of the Related Art
A network-aided GPS receiver receives GPS satellite information from a base station through a data link, which gives the network-aided GPS receiver an advantage over a typical GPS receiver, by providing the network-aided GPS receiver position estimation and reception sensitivity.
Each GPS satellite transmits two microwave carrier signals, L1 and L2. The L1 frequency of 1,575.42 GHz contains modulation data of a coarse acquisition code (C/A code) and a navigation data message with a corresponding phase. The C/A code has a period of 1 ms.
The navigation data message has a 50 bps (bits per second) rate and provides the transmitting GPS satellite""s orbit location, clock offset from GPS time, and, other system parameters.
FIG. 1 illustrates the structure of the navigation data message. The navigation data message is transmitted on a frame basis. Each frame has five sub-frames, where each sub-frame has 300 bits.
A GPS receiver coherently integrates a received GPS satellite signal with an autonomously generated C/A code over 1 ms and compares the integration sum of the GPS satellite signal and the autonomously generated C/A code with a threshold in order to acquire the GPS satellite signal.
A conventional GPS system is limited to only a 1-ms integration period for a C/A code in the coherent integration because it does not know when 50 bps navigation data bits are changed.
On the other hand, a CDMA (Code Division Multiple Access) system time provided by a CDMA or PCS (Personal Communication Services) network is synchronized precisely to a GPS time. Therefore, a network-aided GPS receiver on the CDMA or PCS network can acquire precise GPS time information and coherently integrates a C/A code, which is up to 20 ms and as long as a navigation data bit. As the integration period of the C/A code increases, the reception sensitivity of a GPS signal is improved. The 20-ms integration period increases the reception sensitivity by about 13 dB as compared to the 1-ms integration period.
To increase the C/A code integration period to 20 ms or longer, a GPS receiver must know navigation data bits that operating OPS satellites transmit. These navigation data bits may be derived from previously received navigation data bits but with the following problems:
(1) Ephemeris data, which is 60% of the navigation data bits, is valid for about one hour. This implies that the navigation data bits cannot be obtained every hour. It is impossible to precisely estimate when satellite orbital information is changed at the present;
(2) Almanac data, which is 40% of the navigation data bits is updated every five or six days. Since the entire almanac data is transmitted for 12.5 seconds, corresponding navigation data bits cannot be derived for the length of time; and
(3) If the navigation data bits are transmitted with a time delay or a serving base station does not have the function of providing navigation data bits, it is impossible to derive corresponding navigation data bits.
It is, therefore, an object of the present invention to provide a method of improving GPS signal acquisition search sensitivity by maintaining an integration time for a C/A code and navigation data bits for a predetermined time or longer.
The above object is achieved by utilizing a method to improve the reception performance of a network-aided GPS receiver. It is determined whether the present time is feasible for TLM/HOW (Telemetry Word/Hand-Over Word) coherent integration based on the present CDMA/GPS information. When the TLM/HOW coherent integration is available immediately, a GPS signal acquisition search is performed using the TLM/HOW coherent integration. Then, it is determined whether the signal acquisition search is successful. If the GPS signal acquisition search has failed, requesting navigation data bits to a base station increases a coherent integration period.